Under the current circumstances, the construction of new nuclear power plants is difficult although there is an increasing worldwide requirement for successfully dealing with both a reduction in CO2 emission and an increase in electricity demand. For the extension of the operational life of each existing nuclear power plant operated over along period, it is therefore important to determine the soundness of materials of its equipment and the like such as its reactor and in-core structures and weld zones therein.
As a method for non-destructively detecting and determining the existence or non-existence of cracks formed through IGSCC in the materials of equipment such as a reactor due to conditions such as quality, stress and use environment as well as their dimensions, shapes and the like in the actual equipment, it can be contemplated to apply ultrasonic examination technology, eddy current examination technology or the like.
For the extensive application of the detection and determination of cracks by the above-described technology to positions exposed to various conditions, however, it is necessary to repeat the detection and determination of cracks by using specimens with IGSCC, which occur under various conditions in actual equipment, reproduced experimentally therein and to accumulate data.
Under conditions where a corrosion flaw other than IGSCC to be determined exists in combination, it is difficult to accurately detect and determine a crack formed by IGSCC. There is, accordingly, an outstanding demand for the development of a technique that can experimentally cause only IGSCC to initiate and grow in a short time exclusively at an intended specific area although in actual equipment, IGSCC occurs after an extremely long time under conditions of high-temperature and high-pressure water.
FIGS. 2A and 2B are construction diagrams of a shroud support, which is one of the in-core structures in a pressure vessel in a nuclear power plant, and FIG. 2B is an enlarged view of a section X in FIG. 2A. These figures show the pressure vessel at numeral 100, legs 101 each arranged upright on a bottom part of the pressure vessel 100 via a weld 102, and a support cylinder 103 supported on the legs 101. The support cylinder is connected to each leg 101 via a weld 104. Designated at numeral 105 are support plates arranged between the pressure vessel 100 and the support cylinder 103 and connected to the pressure vessel 100 and the support cylinder 103 via welds 106.
Widely employed as the material of this structure is a nickel-based alloy or stainless steel which can retain corrosion resistance through passivation. With such a material, the application of stress to a sensitized area in the vicinity of grain boundaries (i.e., Cr-depleted area as a result of the deposition of chromium carbides) results in the initiation of a crack, followed by its growth. This is called “IGSCC”.
As a method for causing IGSCC to initiate and grow in a partial specimen of a welded structure in equipment, said welded structure being capable of retaining corrosion resistance through passivation, an accelerated test has been widely used to date. This accelerated test is conducted using as a starting point a simulated defect, which has been formed artificially by cutting or electrical discharge machining, as is or a defect of weld crack, and takes a long time in high-temperature and high-pressure water.
Although not studied for the above purpose, there is ASTM G35 test [Wachenroder's solution, pH<1], which makes use of polythionic acid and may be used as a reference for the simple and easy initiation and growth of IGSCC. This method is known in connection with studies on IGSCC of sensitized stainless steel in desulfurization equipment in the petroleum refinery industry [see Matsushima et al: Boshoku Gijutsu (Corrosion Preventive Technology), 22(4), (1974)].
However, the above-mentioned test simulates the environment of desulfurization equipment. It has, therefore, been reported that the preparation of the test solution is not simple and easy and also that not only IGSCC but also an intergranular corrosion (IGC) attack occurs (see the non-patent publication referred to in the above). Therefore, cracking occurred under test conditions, under which IGC was observed, is considered to be a stress-accelerated IGC phenomenon rather than IGSCC.
As is appreciated from the foregoing, no simple and easy technique has heretofore been established for the development of only IGSCC without IGC or pitting corrosion at room temperature in the atmosphere except for the method that can cause IGSCC to initiate and grow by relying upon a long-time accelerated test in high-temperature and high-pressure water.
An object of the present invention is, therefore, to overcome the above-described drawbacks of the conventional techniques, and to provide a method for simply and easily causing only IGSCC to initiate and grow in a specimen, which can retain corrosion resistance through passivation, in a short time in the atmosphere.